1. Field of the Invention
The present invention relates generally to video image processing, and more particularly, to a noise filter for use in video compression of images.
2. Background Art
With the advent of digital television and streaming media, a number of new techniques have been developed over the recent past to allow video images to be processed and compressed. Moreover, a number of standards have been developed such as those developed by the Moving Pictures Experts Group (MPEG) for coding audio-visual information in a digital compressed format. Various new standards such as MPEG-4 or Advanced Video Coding (AVC) H.264 have been recently developed or are being developed and provide additional parameters to define the processing of digital video data. Additionally, there are a number of different transformation types that are being used to process and compress video data.
One problem with existing approaches is that during the video processing noise is often introduced or added to the original signal. FIG. 1 illustrates a conventional approach 100 to coding audio-visual information into a digital compressed format. A raw video signal is input on line 110 to a motion estimation unit 102 that generates the motion vectors that determine how each motion compensated prediction frame is created from the previous frame. Then a discrete cosine transform (DCT) unit 104 converts the signal into elementary frequency components. This separates the image into parts (or spectral sub-bands) of differing importance (with respect to the image's visual quality). Then quantization unit 106 processes the transformed vectors to produce indices representing the original image data on line 112. The quantization unit 106 takes an input vector and outputs the index of the codeword that offers the lowest distortion. In this case, the lowest distortion is found by evaluating the Euclidean distance between the input vector and each codeword in the codebook. Once the closest codeword is found, the index of that codeword is sent through a channel 112. The variable length coding (VLC) unit 108 receives the indices performs variable length coding to further compress the quantized image. The output of the VLC unit 108 is provide on line 114 and provided the compress image data.
As can be seen from FIG. 1, any one of the couplings between the motion estimation unit 102, DCT unit 104, quantization unit 106, VLC unit 108 or input line 110 can be areas where noise can be introduced into the video signal. Noise often occurs when the source video has background noise. This becomes problematic for a number of reasons. First, the addition of noise to the original signals produces effects on the video output that is displayed. For example, added noise may cause or add flicker to the video image. Second, the noise increased the computational resources required to process the image by introducing additional coefficients requiring additional bits during the encoding process.
Accordingly, there is a need for an improved video processing system that includes a noise filter to reduce the effects of background noise. More particularly there is a need for a noise filter that can modify the video signal between quantization and variable length coding such as under H.264.